On Traditional printers (sheet printers), there are generally two methods in transporting sheets to be printed, that is, a roller-to-roller paper transporting method and a roller suction transporting. If the sheet is transported by a roll-to-roller paper transporting method, requirements for the sheets will be strict. It is necessary to ensure that a leading end of a sheet has entered the second group of rollers when the rear end of the sheet leaves the first group of rollers, so as to ensure that the sheet goes forward normally. This may cause a minimum limit on the length of the sheet in a feeding direction. Moreover, a larger gap between the two rollers will cause that the pulling force is insufficient to send out the sheet, and a smaller gap between the two rollers will cause a larger pressure acting on the sheet, thereby the sheet may be crushed and the strength of the sheet will be affected. If the sheet is transported by the roller suction transporting method, the accuracy of transporting will be poor. Usually, the sheet will be printed in more than two colors, so the registration color accuracy between the two rollers will be relatively poor. In addition, in the suction and transportation process of the prior art, although an air door manually opened and closed is provided on a negative pressure suction box, the air door can only roughly adapt to the width of the sheet while being opened and closed. There are still leakages of air at some suction holes on both sides of the sheet, which causes a poor effect of negative pressure suction.